1. Field of the Invention
The invention relates to a speech and data communication system which is usable on undersea, terrestrial and satellite communication links. More specifically, the invention relates to low-rate encoding (LRE), digital speech interpolation (DSI) system architecture.
2. Description of the Prior Art
In the prior art, the LRE encoders were introduced into the DSI system as shown in FIG. 1. In this configuration, one LRE encoder is required for each input channel. The outputs of the LRE encoders are routed under control of the DSI assignment processor to the appropriate memory locations in the DSI output buffers. If the number of active input channels to be served exceeds the available number of long-distance transmission channels, including overload channels, then the outputs from some of the LRE encoders will be lost and a speech clip will occur. The expression "speech clip" refers to the situation where the leading edge of speech is lost. Since the LRE encoder continues to receive an input signal under this condition, its adaptive quantizer and adaptive predictor section continue to track the input signal. The occurrence of a speech clip prevents transmission of the LRE encoder output to the corresponding LRE decoder at the receiver unit. Consequently, the LRE decoder is unable to adapt its quantizer and predictor sections to match the actions in the LRE encoder. Since this loss of information can be substantial, long-lasting mistracking problems can arise in the LRE decoder causing significant distortion. In order to overcome these problems, the dissipation factor normally used to overcome the effects of bit errors must be substantially increased. This would in turn reduce the unimpaired performance of the LRE codec.
In the prior art when the DSI load increases to the point where overload capacity is required, all LRE codecs are forced to operate in their 3-bit/sample mode. The operating mode of all LRE codecs must be changed together in order to avoid problems associated with selectively modifying the operating mode of a distributed set of LRE codecs for purposes of overload channel formation. Although changing the LRE codecs to their overload mode en masse requires only a single control line from the DSI assignment processor, it has the undesirable effect of causing the entire system to go into overload when only a single overload channel is required.